LDP IGP synchronization for broadcast networks

ABSTRACT

A network element that has a broadcast interface to a broadcast network becoming operational determines whether at least one alternate path exists to the broadcast network. The broadcast interface is to carry traffic on a label switched path. If an alternate path exists, the broadcast interface will not be advertised to the broadcast network until label distribution protocol (LDP) is operational with all neighbors on the broadcast interface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.61/157,501, filed Mar. 4, 2009, which is hereby incorporated byreference.

BACKGROUND

1. Field

Embodiments of the invention relate to the field of networking; and morespecifically to LDP (Label Distribution Protocol) IGP (Interior GatewayProtocol) synchronization for broadcast networks.

2. Background

LDP (described in “LDP Specification”, RFC 5036, October 2007), which isused to establish LSPs (label switched paths) to destinations, typicallyrelies on IGP (e.g., Open Shortest Path First (OSPF) (defined in “OSPFVersion 2”, STD 54, RFC 2328, April 1998), Intermediate system toIntermediate system (IS-IS) (defined in “Intermediate system tointermediate system intra-domain-routing routine information exchangeprotocol for use in conjunction with the protocol for providing theconnectionless-mode Network Service (ISO 8473)” ISO standard 10589,1992), etc.) to provide the underlying routing information (e.g., thecost metric between the hops in the network). Even though LDP typicallyrelies on the IGP protocol, they are independent from each other. IGPmay be operational on a link prior to LDP becoming operational on thatlink which can lead to packet loss.

The request for comments (RFC) 5443, “LDP IGP Synchronization”, March2009 (hereinafter “LDP IGP Synchronization”) describes a mechanism todiscourage links from being used for IP forwarding if LDP is not fullyoperational on that link. RFC 5443 describes that when LDP is not fullyoperational on a given link (e.g., all the label bindings have not yetbeen exchanged), IGP will advertise the link with maximum cost todiscourage traffic from being sent over the link. When LDP on the linkbecomes operational (e.g., all label bindings have been exchanged), IGPadvertises the link with the correct cost.

On broadcast links (more than one LDP/IGP peer on the same link), IGPadvertises a common cost to the broadcast link, rather than a separatecost to each peer. A broadcast pseudo-node may be implemented in anetwork to reduce the number of links in the shortest path first (SPF)(e.g., open shortest path first (OSPF), constrained shortest path first(CSPF)) database of each network element. Each network element formsadjacencies with the broadcast pseudo-node and advertises a link andcost to the broadcast pseudo-node and the broadcast pseudo-nodeadvertises a link and cost to each network element.

For broadcast links, the RFC 5443 “LDP IGP Synchronization” describesthat the mechanism can be applied to the link as a whole and not anindividual peer. Advertising maximum cost to the link as a whole maycause sub-optimal traffic diversion and/or black-holing of traffic(e.g., VPN traffic).

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by referring to the followingdescription and accompanying drawings that are used to illustrateembodiments of the invention. In the drawings:

FIG. 1 illustrates an exemplary broadcast network according to oneembodiment of the invention;

FIG. 2 is a block diagram of an exemplary network element configured forLDP-IGP synchronization for broadcast networks according to oneembodiment of the invention;

FIG. 3 is a flow diagram illustrating exemplary operations for LDP-IGPsynchronization for broadcast networks according to one embodiment ofthe invention;

FIG. 4 is a flow diagram illustrating exemplary operations when LDP-IGPsynchronization is complete according to one embodiment of theinvention;

FIG. 5A illustrates an exemplary local IGP interface structure accordingto one embodiment of the invention;

FIG. 5B illustrates an exemplary link state database according to oneembodiment of the invention;

FIG. 6 is a flow diagram illustrating exemplary operations when anetwork element is ready to update its link-state advertisement with aninterface to a broadcast network according to one embodiment of theinvention; and

FIG. 7 illustrates an exemplary network element used in some embodimentsof the invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. In other instances, well-knowncircuits, structures and techniques have not been shown in detail inorder not to obscure the understanding of this description. Those ofordinary skill in the art, with the included descriptions, will be ableto implement appropriate functionality without undue experimentation.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

In the following description and claims, the terms “coupled” and“connected,” along with their derivatives, may be used. It should beunderstood that these terms are not intended as synonyms for each other.“Coupled” is used to indicate that two or more elements, which may ormay not be in direct physical or electrical contact with each other,co-operate or interact with each other. “Connected” is used to indicatethe establishment of communication between two or more elements that arecoupled with each other.

The techniques shown in the figures can be implemented using code anddata stored and executed on one or more electronic devices (e.g., anetwork element, etc.). Such electronic devices store and communicate(internally and/or with other electronic devices over a network) codeand data using machine-readable media, such as machine-readable storagemedia (e.g., magnetic disks; optical disks; random access memory; readonly memory; flash memory devices; phase-change memory) andmachine-readable communication media (e.g., electrical, optical,acoustical or other form of propagated signals—such as carrier waves,infrared signals, digital signals, etc.). In addition, such electronicdevices typically include a set of one or more processors coupled to oneor more other components, such as a storage device, one or more userinput/output devices (e.g., a keyboard, a touchscreen, and/or adisplay), and a network connection. The coupling of the set ofprocessors and other components is typically through one or more bussesand bridges (also termed as bus controllers). The storage device andsignals carrying the network traffic respectively represent one or moremachine-readable storage media and machine-readable communication media.Thus, the storage device of a given electronic device typically storescode and/or data for execution on the set of one or more processors ofthat electronic device. Of course, one or more parts of an embodiment ofthe invention may be implemented using different combinations ofsoftware, firmware, and/or hardware.

As used herein, a network element (e.g., a router, switch, bridge, etc.)is a piece of networking equipment, including hardware and software,that communicatively interconnects other equipment on the network (e.g.,other network elements, computer end stations, etc.). Some networkelements are “multiple services network elements” that provide supportfor multiple networking functions (e.g., routing, bridging, switching,Layer 2 aggregation, and/or subscriber management), and/or providesupport for multiple application services (e.g., data, voice, andvideo).

Within certain network elements, multiple “interfaces” may beconfigured. As used herein an interface is a connection between anetwork element and one of its attached networks. An interface hasinformation associated with it that is typically obtained fromunderlying lower level protocols and the routing protocol (e.g., an IPaddress and mask). An interface is sometimes referred to as a link. Abroadcast interface is an interface that is connected with a broadcastnetwork.

A method and apparatus for LDP-IGP synchronization for broadcastnetworks is described. In one embodiment of the invention, when a linkto a broadcast network comes up, the network element whose link iscoming up will postpone advertising the link to the broadcast networkuntil LDP is operational with all neighbors on that broadcast link (allbroadcast neighbors). For example, that network element determineswhether that link is the only path to the broadcast network. If thatlink is the only path to the broadcast network, the interfacecorresponding to that link is marked as a cut-edge interface. When thenetwork element is ready to update its link-state advertisement (LSA)with the link to the broadcast network, it will postpone adding thatlink to its LSA if the corresponding interface is not a cut-edgeinterface and LDP is not operational with all neighbors on thatinterface. After LDP becomes operational with all neighbors on thatinterface (e.g., LDP becomes synchronized with IGP), the link will beadded to the LSA and will be transmitted to the broadcast network.

As used herein, the term “link state advertisements” and “LSA” isprotocol agnostic. For example, if the IGP protocol is Open ShortestPath First (OSPF), then the link state advertisement may be aRouter-LSA. If the IGP protocol is intermediate system to intermediatesystem (IS-IS), then the link state advertisement may be a Link StatePDU.

In one embodiment, if the IGP protocol is Open Shortest Path First(OSPF), the Router-LSA is not updated with a Link Type 2 (link totransit network) for the subnet until LDP is operational with allnetwork elements on that subnet.

In one embodiment, if the IGP protocol is intermediate system tointermediate system (IS-IS), the Link State PDU is not updated with anIS Reachability TLV (or an Extended IS Reachability TLV) to thebroadcast network until LDP is operational with all network elements.

FIG. 1 illustrates an exemplary broadcast network according to oneembodiment of the invention. The network 100 includes the networkelements 105, 110, 130, 140, 145, 150, 155, 160, 165, and 170, and thebroadcast pseudo-node 120. The network elements 110, 130, 140, 145, and150 are each directly coupled with the broadcast pseudo-node 120. Itshould be understood that the topology of the network 100 is exemplary,and other topologies may be used in embodiments of the invention. Forexample, in some embodiments, a broadcast pseudo-node is not used.

In one embodiment, the network elements 105, 160, 165, and 170 areprovider edge network elements and the network 100 is a MultiprotocolLabel Switching (MPLS) network with Virtual Private Network (VPN)applications. As one example, the network element 105 may have a LDP LSPpath to the network elements 160, 155, 170, and/or 165. It should beunderstood that other LDP LSP paths may be established in the network100.

The network elements 110, 130, 140, 145, and 150 each advertise theircorresponding interface's cost to the broadcast pseudo-node 120. Forpurposes of explanation, the network element 110 will advertise a costof 1 on the broadcast interface 114, the network element 130 willadvertise a cost of 1 on the broadcast interface 132, the networkelement 140 will advertise a cost of 1 on the broadcast interface 142,the network element 145 will advertise a cost of 1 on the broadcastinterface 146, and the network element will advertise 152 will advertisea cost of 1 on the broadcast interface 152. In addition, exemplarypurposes, each other link in the network 100 has a cost of 1 with theexception of the link between the network element 110 and the networkelement 165 which has a cost of 10. The broadcast pseudo-node 120 alsoadvertises its interfaces to its neighbors (e.g., through the interfaces122, 124, 126, and 127) with a cost of zero. It should be understoodthat the interfaces illustrated in FIG. 1 are exemplary and otherinterfaces are not illustrated in order not to confuse understanding ofthe invention.

In one interpretation of the RFC 5443 “LDP IGP Synchronization” asapplied to broadcast networks, when a new network element is discoveredon the broadcast network, all the network elements with direct links tothe broadcast network advertise maximum cost to the broadcast network.For example, with reference to FIG. 1, if the broadcast interface 132 ofthe network element 130 becomes operational (e.g., the network element130 was added to the network while the network elements 110, 140, 145,and 150 are each already connected with the broadcast pseudo-node 120)and detected by the network elements 110, 140, 145, 150, those networkelements each start advertising maximum cost to the broadcast network(the network elements 110, 132, 140, 145, and 150 advertise maximum coston the broadcast interfaces 114, 132, 142, 146, and 152 respectively).Since maximum cost is greater than the cost of 10, traffic between thenetwork element 105 and the network element 165 will be diverted to thesub-optimal path of network element 105 to the network element 110 tothe network element 165 (instead of the optimal path of network element105 to network element 110 to the network element 150 to the networkelement 165). Thus, using the interpretation of the RFC “LDP IGPSynchronization” that has each network element advertising a maximumcost to the broadcast network may result in sub-optimal trafficdiversion.

In addition, as a result of applying the mechanism described in the RFC5443 “LDP IGP Synchronization” for broadcast networks, traffic may beblack-holed (continually dropped) at a network element due to eachbroadcast network element advertising a maximum cost. For example, usingthe network topology of FIG. 1 and the same costs of the links in theprevious example, the network element 105 transmits VPN traffic to thenetwork element 160 prior to the interface 132 becoming operational onthe network element 130 and the LDP LSP carrying that VPN traffic willtake the following path: network element 105 to the network element 110to the network element 140 to the network element 155 to the networkelement 160 (this is the only path possible in the topology). After theinterface 132 becomes operational, it should be understood that anadditional possible LDP LSP from the network element 105 to the networkelement 160 will exist once LDP peering has been established along thatpath (network element 105 to the network element 110 to the networkelement 130 to the network element 160). Once established, this LDP LSPwill be favored over the previous LSP because it will have a lower cost(3 versus 4 in the example). However, since each link of the broadcastnetwork will be advertised as a maximum cost, the path through thenetwork element 130 will have a lower cost than the path through thenetwork elements 140 and 155. As a result, the network element 110 willswitch its nexthop to the network element 160 from the network element140 to the network element 130 before an LDP LSP has been establishedthrough the network element 130. Consequently, the network element 110will try, and fail, to transmit VPN traffic to the network element 160through the network element 130 until that LDP LSP has been established(e.g., the network element 110 will not have the proper labels toprocess the VPN traffic). Thus, the VPN traffic will be black-holed atthe network element 110. The amount of traffic loss in this case is atleast the order of the time it takes for the LDP LSP to becomeoperational. Although the topology in FIG. 1 is rather simple, it shouldbe understood that in some topologies this can be of the order ofseveral minutes (which can violate carrier class reliability metrics).

In another interpretation of the RFC 5443 “LDP IGP Synchronization” asapplied to broadcast networks, when a new network element is discoveredon the broadcast network, only that network element transmits a maximumcost to the broadcast network and the other network elements transmittheir normal cost. However, this interpretation also suffers from thepossibility of black-holing traffic. It should be understood thatalthough the network element 130 advertises a maximum cost to thebroadcast network, this only affects traffic being transmitted by thenetwork element 130 and it will not affect traffic being received by thenetwork element 130. Thus, from the point of view of the network element110, the link between the network element 110 and the network element130 will have a cost of 1. Using the same example as above and since IGPwill converge faster than LDP, the network element 110 will try, andfail, to transmit VPN traffic to the network element through the networkelement 130 until that LDP LSP has been established. Thus, similar tothe above example, the VPN traffic will be black-holed at the networkelement 110.

In contrast, as will be described later herein, embodiments of theinvention allow for LDP-IGP synchronization in broadcast networkswithout sub-optimally diverting traffic or black-holing traffic.

FIG. 2 is a block diagram of an exemplary network element configured forLDP-IGP synchronization for broadcast networks according to oneembodiment of the invention. While FIG. 2 illustrates the networkelement 130, it should be understood that one or more of the networkelements in the network 100 may include similar features. As illustratedin FIG. 2, the network element 130 includes the control plane 210coupled with the data plane 260. The control plane 210 includes thecommand line interface 230, the IGP module 220, the interface statemanager 235, the LDP module 240, and the label manager 250. The IGPmodule 200 includes the SPF and cut-edge detection module 280 and theLDP-IGP synchronization LSA module 285. The IGP module 220 manages theneighbor adjacency table 222, the link state database 224, the local IGPRIB (routing information base) 226, and the local IGP interfacestructure 228.

The IGP module 220 may receive LDP-IGP synchronization for broadcastnetworks configuration parameters from the command line interface 230.For example, a network administrator may use the command line interface230 to configure the LDP-IGP synchronization for broadcast networks onthe network element 130 (e.g., whether the LDP-IGP synchronization forbroadcast networks is enabled, one or more interfaces to monitor forLDP-IGP synchronization for broadcast networks, etc.). In anotherembodiment, the LDP-IGP synchronization for broadcast networks mechanismis installed for each broadcast interface on the network element 130.

The interface state manager 235 manages the interfaces of the networkelement 130, including the interfaces 132 and 134. For example, theinterface state manager 235 detects when an interface is operational.The interface state manager 235 is coupled with the IGP module 220. TheIGP module 220 registers those broadcast interfaces (e.g., theinterfaces as specified during configuration) with the interface statemanager 235. The interface state manager 235 notifies the IGP module 220upon a state change of one of those registered interfaces (e.g., abroadcast interface becoming operational, a broadcast interface goingdown, etc.). The IGP module 220 may then update the local IGP interfacestructure 228 with those interfaces.

The IGP module 220 establishes and maintains neighbor adjacencies withother network elements in the network 100. For example, the IGP module220 transmits and receives hello packets from other network elements inthe network 100. From these hello packets, the IGP module creates andmaintains the neighbor adjacency table 222.

The IGP module 220 also transmits and receives link-state information(typically in the form of link-state advertisements (LSAs)) to constructa topology of the network 100. From the LSAs it receives and transmits,the IGP module 220 creates and maintains the link state database 224(thus the link state database 224 is a representation of the networktopology of the network 100). FIG. 5B illustrates an exemplary linkstate database 224 according to one embodiment of the invention. Asillustrated in FIG. 5B, the link state database 224 includes a Fromfield 530, a To field 540, and a Link field 550.

The IGP module 220 also includes the SPF and cut-edge detection module280 to determine the optimum paths to the destinations of the link statedatabase 224 (thus, the SPF algorithm is applied to the information ofthe link state database 224). The resulting routes are added to thelocal IGP RIB 226, which are then programmed to one or more FIBs(forwarding information bases) on the data plane 260. For example, theroutes in the IGP RIB 226 are programmed to one or more packetprocessing units of one or more line cards in the network element 130.

In addition to determining the optimum paths to the destinations in thelink state database 224, the SPF and cut-edge detection module 280determines whether interfaces are broadcast cut-edge interfaces. Abroadcast cut-edge interface is an interface to the broadcast networkthat represents the only path to that broadcast network. In other words,besides connecting to the broadcast network through the broadcastinterface, if an alternate path to the broadcast network does not exist,that broadcast interface is a cut-edge interface. For example, withreference to FIG. 1, the interface 132 is not a cut-edge interfacebecause an alternate IGP path exists to the broadcast pseudo-node 120(network element 130 to the network element 160 to the network element155 to the network element 140 to the broadcast pseudo-node 120). Theinterface 146 is a cut-edge interface because an alternate IGP path doesnot exist to the broadcast pseudo-node 120. If an interface is acut-edge interface, a cut-edge property is set for that interface in thelocal IGP interface structure 228.

In one embodiment of the invention, the SPF and cut-edge detectionmodule 280 makes the determination of a broadcast cut-edge interface byaugmenting the step of SPF calculation that adds a network prefix to thelocal IGP RIB 226 to include an additional check whether that prefixbelongs to an interface that is down (e.g., if the prefix matches a downinterface, the prefix will be viable from somewhere else and theinterface will not be a cut-edge) or is a stub network through whichtraffic cannot be further transited beyond. Of course, other methods maybe used to determine whether alternate paths to the broadcast networkexist.

As previously described, the IGP module 220 transmits and receives LSAsto construct the topology of the network 100. The IGP module 220includes the LDP-IGP synchronization LSA module 285 which excludes linksfrom the LSAs whose corresponding broadcast interfaces are not cut-edgeinterfaces until LDP becomes operational to all neighbors on thatinterface. In other words, for a broadcast interface, if there is analternate path to the broadcast network, the LDP-IGP synchronization LSAmodule 285 will exclude that link corresponding to the broadcastinterface from its LSAs until the LDP and IGP have synchronized (e.g.,when LDP is operational with all neighbors on that broadcast interface).In one embodiment, if LDP is not fully operational for a broadcastinterface, a suppress link from LSA flag is set in the local IGPinterface structure 228.

FIG. 5A illustrates an exemplary local IGP interface structure 228according to one embodiment of the invention. As illustrated in FIG. 5A,the local IGP interface structure 228 includes the interface field 510which identifies a local IGP interface, the cut-edge interface propertyfield 515 which indicates whether an interface is a cut-edge interface,and the suppress link from LSA flag 520 which indicates whether theinterface should be suppressed from the LSA.

The LDP module 240 negotiates labels with the other network elements inthe network 100. In one embodiment, the LDP module 240 determines whenLDP is fully operational for a particular broadcast interface andnotifies the IGP module 220 that LDP is fully operational for thatinterface. The IGP module 220 may then clear the suppress link from LSAflag 520 if it has been set (if the interface was not a cut-edgeinterface) and advertise that link to the other network elements. TheLDP module 240 is also coupled with the label manager 250, which createsand maintains the LSP structure(s) 255 which, among other things, createand manage the labels for the LSPs. The labels are programmed to one ormore label forwarding information bases (LFIBs) in the data plane 260.For example, the labels stored in the LSP structure(s) 255 areprogrammed to one or more packet processing units of one or more linecards in the network element 130.

It should be understood that in some embodiments cut-edge interfaces arenot excluded from the LSA since they are the only route to the broadcastnetwork. Thus, since there is no other path for the traffic to flow, thetraffic for any LSP will only start after LDP is operational.

FIG. 3 is a flow diagram illustrating exemplary operations for LDP-IGPsynchronization for broadcast networks according to one embodiment ofthe invention. The operations of FIG. 3 will be described with referenceto the exemplary embodiments of FIGS. 1 and 2. However, it should beunderstood that the operations of FIG. 3 can be performed by embodimentsof the invention other than those discussed with reference to FIGS. 1and 2, and the embodiments discussed with reference to FIGS. 1 and 2 canperform operations different than those discussed with reference to FIG.3.

At operation 310, a neighbor up event is detected on a broadcastinterface. For example, the IGP module 220 may detect neighbor up eventby receiving a hello packet message from a network element on theinterface 132 which is not represented in the neighbor adjacency table222, where the hello packet includes an identifier of the networkelement 132, and their LSA information is exchanged and synchronized (itshould be understood that this link state advertisement information doesnot include the interface to the broadcast network, i.e., the broadcastinterface 132). Flow moves from block 310 to block 320, where the IGPmodule 220 determines if the new neighbor is the first neighbor on theinterface. For example, if the neighbor adjacency table 222 does notinclude an entry for the interface in the neighbor adjacency table 222,then the new neighbor is the first neighbor on the interface.

If the discovered neighbor is the first neighbor on the interface, thenflow moves to block 335, otherwise flow moves to block 350. At block315, an interface to the broadcast network is detected as up. Forexample, the interface state manager 235 detects that the interface 132is operational. Flow moves from block 315 to block 335, where the IGPmodule 220 determines whether the detected interface 132 is a cut-edgeinterface to the broadcast pseudo-node 120. For example, the IGP module220 accesses the local IGP interface structure 228 to check the cut-edgeproperty of the interface 132. With reference to FIG. 1, the interface132 is not a cut-edge interface (and the cut-edge property will indicateas such) since an alternate path exists to the broadcast pseudo-node120. If the interface is not a cut edge, flow moves to block 340,otherwise flow moves to block 350.

At block 340, the IGP module 220 determines if LDP-IGP synchronizationis complete for the interface 132. For example, the IGP module 220accesses the local IGP interface structure 228 to check the status ofthe suppress link to LSA flag for the interface 132. The flag willindicate whether IGP module 220 will treat LDP and IGP as beingsynchronized (e.g., if the flag is set, then the IGP module 220 treatsLDP and IGP as not being synchronized and if the flag is not set, thenthe IGP module 220 treats LDP and IGP as being synchronized). If LDP-IGPsynchronization is complete, flow moves to block 325; otherwise flowmoves to block 345. At block 345, since LDP-IGP synchronization is notcomplete and the interface is not a cut-edge interface, the suppresslink to LSA flag is set. Flow moves from block 345 to block 350 wereprocessing is continued and only those links which are not suppressed(as indicated by the suppress link LSA flag) are advertised to thebroadcast network.

For example, the IGP module 220 postpones transmitting an LSA thatincludes the broadcast interface 132 to the broadcast pseudo-node 120until LDP is operational on all neighbors on the interface 132. Thus,unlike the RFC “LDP IGP Synchronization” which would advertise the linkwith maximum cost until LDP is operational, in embodiments of theinvention the link is not advertised until LDP is operational. By notadvertising the link until LDP and IGP are synchronized, the othernetwork elements in the network 100 cannot use that link because thebidirectional connectivity check would fail until LDP and IGP aresynchronized. Thus, unlike the examples previously discussed withreference to the RFC “LDP IGP Synchronization,” traffic will not beredirected to sub-optimal links and traffic will not be black-holedusing embodiments of the invention.

FIG. 4 is a flow diagram illustrating exemplary operations when LDP-IGPsynchronization is complete according to one embodiment of theinvention. The operations of FIG. 4 will be described with reference tothe exemplary embodiments of FIGS. 1 and 2. However, it should beunderstood that the operations of FIG. 4 can be performed by embodimentsof the invention other than those discussed with reference to FIGS. 1and 2, and the embodiments discussed with reference to FIGS. 1 and 2 canperform operations different than those discussed with reference to FIG.4.

At block 410, responsive to the LDP module 240 exiting from the LDP-IGPsynchronization process for an interface, the LDP module 240 sends aLDP-IGP sync notification message to the IGP module 220. In oneembodiment, the LDP-IGP synchronization process will end upon aconfigurable timer that allows LDP session establishment expiring. Forexample, based on empirical data, the network administrator mayconfigure a timer which provides a worst case (or best guess) of thetime it should take for LDP to become operational on all neighbors ofthe interface. The configuration value for the timer may be included inthe LDP-IGP synchronization for broadcast networks configurationparameters received by the IGP module 220.

In another embodiment the network element 130 may implement the LDPEnd-of-LIB mechanism as specified in IETF draft “LDP End-of-LIB:draft-ietf-mpls-end-of-lib-03.txt”, January 2009, to determine when theLDP-IGP synchronization process exits. For example, in LDP End-of-LIB,each LDP peer (neighbor) may signal completion of its labeladvertisements following session establishment. After receiving allcompletion signals from each LDP peer, the LDP will be operational withall neighbors and the LDP-IGP synchronization process will exit.

Flow moves from block 410 to block 415, where the IGP module 220determines if the suppress link to the LSA flag 520 is set for theinterface. If the flag is not set, then flow moves to block 420 wherealternative action is taken (e.g., nothing needs to be performed). Ifthe flag is set, then flow moves to block 425 where the IGP module 220clears the suppress link to the LSA flag 520 for the interface. Flowmoves from block 425 to block 430.

In one embodiment of the invention, an interface will be excluded fromthe link state database 224 if the suppress link to LSA flag is set.Thus, at block 430, the IGP module 220 updates the link state database224 with the interface. Flow moves from block 430 to block 435, wherethe IGP module 220 runs a SPF algorithm on the link state database topossibly update the local IGP RIB 226 with a better route. Flow nextmoves to block 440, where the data plane (e.g., the FIB(s) 270) isprogrammed with the updated route information. Flow next moves to block445, where the IGP module 220 includes the interface in its LSA and thenflow moves to block 450 where the IGP module 220 transmits the LSAincluding the interface to its neighbors.

FIG. 6 is a flow diagram illustrating exemplary operations when anetwork element is ready to update its link-state advertisement with aninterface to a broadcast network according to one embodiment of theinvention. The operations of FIG. 6 will be described with reference tothe exemplary embodiments of FIG. 2. However, it should be understoodthat the operations of FIG. 6 can be performed by embodiments of theinvention other than those discussed with reference to FIG. 2, and theembodiments discussed with reference to FIG. 2 can perform operationsdifferent than those discussed with reference to FIG. 6.

At block 610, the LDP-IGP synchronization LSA module 285 begins anupdate link state advertisement process. The update link stateadvertisement process will include an attempt to add a broadcastinterface to the broadcast network (e.g. the broadcast pseudo-node 120).Flow moves from block 610 to block 620, where the LDP-IGPsynchronization LSA module 285 determines whether the suppress link toLSA flag is set for the interface. If the flag is not set, then flowmoves to block 630 where the interface is included in the link stateadvertisement. If the IGP is OSPF, the Router-LSA is updated with a LinkType 2 (link to transit network) for the subnet. If the IGP is ISIS, theLink State PDU is updated with an IS Reachability TLV (or an Extended ISReachability TLV). If the flag is set, then flow moves to block 640where that interface will be excluded from the link state advertisement.

Unlike the RFC 5443 “LDP IGP Synchronization,” the LDP-IGPsynchronization for broadcast networks mechanism described herein is notrequired to be implemented on each of the network elements 110 130, 140,145, and 150 in the network 100. In some embodiments, at a minimum, onlya network element that is being added to the broadcast networkimplements the LDP-IGP synchronization for broadcast networks mechanismdescribed herein. Thus, the LDP-IGP synchronization for broadcastnetworks mechanism described herein may be introduced into a network ina gradual manner without any backward compatibility issues.

Furthermore, unlike the RFC 5443 “LDP IGP Synchronization” whichrequires that each network element advertising a maximum cost to thebroadcast network revert back to advertising their normal cost after LDPpeering between all the network elements on the broadcast network isoperational (which is a non-trivial process that requires coordinationbetween all the network elements), embodiments of the invention do notrequire any action to be taken by the other network elements other thanthe network element whose link to the broadcast network is coming up.

FIG. 7 illustrates an exemplary network element used in some embodimentsof the invention. As illustrated in FIG. 7, the network element 130includes the control cards 715 and 720 (e.g., one control card is activethe other is a backup), the resource cards 725A-725N, and the line cards730A-730N. It should be understood that the architecture of the networkelement 130 illustrated in FIG. 7 is exemplary, and differentcombinations of cards may be used in other embodiments of the invention.

Each of the cards illustrated in FIG. 7 include one or more processorsand one or more memories. For example, the line cards 730A-730Btypically include one or more packet processing units to process packetsincluding forwarding and/or switching packets at high speed, and includeone or more memories to store a forwarding information base (sometimesreferred to as a routing table) and a label forwarding information base.The control cards 715 and 720 also include one or more processors toperform signaling, routing (including creation of and/or management ofrouting tables), connection setup, session setup, etc. For example,among other things, the control card 715 execute instructions stored inmemory to execute the IGP module 220 and the LDP module 240.

While the flow diagrams in the figures show a particular order ofoperations performed by certain embodiments of the invention, it shouldbe understood that such order is exemplary (e.g., alternativeembodiments may perform the operations in a different order, combinecertain operations, overlap certain operations, etc.)

While the invention has been described in terms of several embodiments,those skilled in the art will recognize that the invention is notlimited to the embodiments described, can be practiced with modificationand alteration within the spirit and scope of the appended claims. Thedescription is thus to be regarded as illustrative instead of limiting.

1. A method for LDP-IGP (label distribution protocol-interior gatewayprotocol) synchronization on a broadcast network performed on a networkelement, comprising: detecting that a broadcast interface of the networkelement has become operational, wherein the broadcast interface will beused to carry traffic through a label switched path (LSP) between thenetwork element and the broadcast network; determining that there is atleast one alternate path to reach the broadcast network besides throughthe broadcast interface; and postponing including the broadcastinterface in a link state advertisement (LSA) until label distributionprotocol (LDP) is operational with all neighbors on that broadcastinterface.
 2. The method of claim 1, further comprising: responsive todetermining that LDP is operational with all neighbors on that broadcastinterface, including that broadcast interface in a LSA and transmittingthat LSA to the broadcast network.
 3. The method of claim 1, furthercomprising: setting a suppress link to LSA flag for the broadcastinterface until LDP is operational with all neighbors on that broadcastinterface; and wherein postponing of including the broadcast interfacein the LSA includes checking the suppress link to LSA flag when updatinga LSA to determine whether to add the broadcast interface to the updatedLSA.
 4. The method of claim 1, further comprising: postponing updating alink state database (LSDB) of the network element with the broadcastinterface until LDP is operational with all neighbors on the broadcastinterface.
 5. The method of claim 1, wherein determining that there isat least one alternate path to reach the broadcast network includes thefollowing: during execution of a shortest path first (SPF) algorithm,determining that a network prefix associated with the broadcastinterface belongs to a down interface.
 6. The method of claim 1, whereinLDP is determined to be operational with all neighbors after a LDP-IGPsynchronization timer has expired.
 7. The method of claim 1, wherein thenetwork element is implementing Open Shortest Path First (OSPF), andwherein postponing including the broadcast interface in an LSA includespostponing updating a router-LSA with a Link Type 2 for the broadcastnetwork until LDP is operational with all neighbors on that broadcastinterface.
 8. The method of claim 1 wherein the network element isimplementing intermediate system to intermediate system (IS-IS), andwherein postponing including the broadcast interface in a LSA includespostponing updating a Link State PDU with an IS Reachability TLV for thebroadcast network until LDP is operational with all neighbors on thatbroadcast interface.
 9. The method of claim 1, wherein the traffic isVirtual Private Network (VPN) traffic.
 10. A network element to performLDP-IGP (label distribution protocol-interior gateway protocol)synchronization on a broadcast network, comprising: one or moreprocessors to execute instructions; and one or more memories coupledwith the processor, the one or more memories having instructions that,when executed cause the one or more processors to perform operationsincluding, detecting that a broadcast interface of the network elementhas become operational, wherein the broadcast interface will be used tocarry traffic through a label switched path (LSP) between the networkelement and the broadcast network, determining that there is at leastone alternate path to reach the broadcast network besides through thebroadcast interface, and postponing including the broadcast interface ina link state advertisement (LSA) until label distribution protocol (LDP)is operational with all neighbors on that broadcast interface.
 11. Thenetwork element of claim 10, wherein the one or more memories furtherhas instructions which when executed cause the one or more processors toperform the following operations: responsive to determining that LDP isoperational with all neighbors on that broadcast interface, includingthat broadcast interface in a LSA and transmitting that LSA to thebroadcast network.
 12. The network element of claim 10, wherein the oneor more memories further has instructions which when executed cause theone or more processors to perform the following operations: setting asuppress link to LSA flag for the broadcast interface until LDP isoperational with all neighbors on that broadcast interface; and whereinpostponing of including the broadcast interface in the LSA includeschecking the suppress link to LSA flag when updating a LSA to determinewhether to add the broadcast interface to the updated LSA.
 13. Thenetwork element of claim 10, wherein the one or more memories furtherhas instructions which when executed cause the one or more processors toperform the following operations: postponing updating a link statedatabase (LSDB) of the network element with the broadcast interfaceuntil LDP is operational with all neighbors on the broadcast interface.14. The network element of claim 10, wherein determining that there isat least one alternate path to reach the broadcast network includes thefollowing: during execution of a shortest path first (SPF) algorithm,determining that a network prefix associated with the broadcastinterface belongs to a stub network.
 15. The network element of claim10, wherein LDP is determined to be operational with all neighbors aftera LDP-IGP synchronization timer has expired.
 16. The network element ofclaim 10, wherein the network element is implementing Open Shortest PathFirst (OSPF), and wherein postponing including the broadcast interfacein an LSA includes postponing updating a router-LSA with a Link Type 2for the broadcast network until LDP is operational with all neighbors onthat broadcast interface.
 17. The network element of claim 10 whereinthe network element is implementing intermediate system to intermediatesystem (IS-IS), and wherein postponing including the broadcast interfacein a LSA includes postponing updating a Link State PDU with an ISReachability TLV for the broadcast network until LDP is operational withall neighbors on that broadcast interface.
 18. A network element toperform label distribution protocol (LDP)-interior gateway protocol(IGP) synchronization for broadcast networks, comprising: a set of oneor more processors on a control card of the network element; an IGPmodule, that when executed by the set of processors, cause the set ofprocessors to perform the following: determine whether broadcastinterfaces of the network element are cut-edge interfaces to broadcastnetworks, wherein a cut-edge interface is an interface to a broadcastnetwork and no other paths to that broadcast network exist, and postponeincluding those broadcast interfaces that are not cut-edge interfaces inlink state advertisements until LDP is operational with all neighbors onthose broadcast interfaces; and an LDP module, that when executed by theset of processors, cause the set of processors to exchange labelbindings with neighbors of the network element for the broadcastinterfaces.
 19. The network element of claim 18, wherein the IGP moduleis further to, when executed by the set of processors, postpone updatinga link state database with those broadcast interfaces that are notcut-edge interfaces until LDP is operational with all neighbors on thosebroadcast interfaces.
 20. The network element of claim 18, wherein theIGP module is further, that when executed by the set of processors, toperform a shortest path first (SPF) algorithm that includes determiningwhether broadcast interfaces are cut-edge interfaces.
 21. The networkelement of claim 18, wherein the IGP module is further to, that whenexecuted by the set of processors, include those broadcast interfacesthat are cut-edge interfaces in link state advertisements regardlesswhether LDP is operational with all neighbors on those broadcastinterfaces.